Digital data communication systems oftentimes transmit information in the form of pulses. A stream of information (or digital bits) is conveyed as a stream of pulses. In one simple embodiment, a digital "1" is transmitted as a positive pulse, while a digital "0" is transmitted as a negative pulse. Each pulse lasts for a duration of time inversely proportional to the bit rate of the system. Thus, for a simple binary system (i.e., one bit per baud) transmitting 1 Mb/s, then the time duration for each bit is 1 microsecond. The incoming signal is sampled once per baud to determine the digital bit to be assigned to that baud. As the baud rate of the system increases, the baud interval decreases and it becomes necessary to insure that the received signal is being sampled at the proper point in time within each baud. The baud interval is often denoted T, and the baud rate denoted 1/T.
One critical component of a data communications system is a symbol lock detector, which is used to determine when the proper sampling point for each baud has been achieved. Another critical component in a communications system is a phase detector, which is used in a phase locked loop (PLL) to achieve the zero phase point of the received signal.
That is, for a pulse signal representation, the points where the signal waveform crosses the horizontal axis (i.e., zero phase) are determined.
In a phase detector for a T/2 (two times oversampling) system, the received signal is sampled at a reference location (and T/2 (half the baud interval) later, i.e., the "T/2" location. One of the sample points is used to determine the actual data value, while both samples are used to determine the correct phase of the received signal.
Conventional two times oversampling symbol lock detectors need to sample an incoming signal at the T/4 (one-quarter baud interval after the reference location) and 3T/4 (three-quarters of the baud interval after the reference location) marks for proper operation. However, the T/4 and 3T/4 sample points are generally not available and must be especially determined in addition to the reference and T/2 sample points. The requirement of having to sample the received signal an additional two times (T/4 and 3T/4) adds to the cost and complexity of the receiver circuit.